Add document for aws_db_instance_invalid_type (#63)

ref: #61 (comment)

Co-authored-by: kondo takeshi <[email protected]>

Author: chaspy

docs/rules/README.md

@@ -14,7 +14,7 @@ These rules warn of possible errors that can occur at `terraform apply`. Rules m
 |[aws_db_instance_invalid_engine](aws_db_instance_invalid_engine.md)||
 |aws_db_instance_invalid_option_group|✔|
 |aws_db_instance_invalid_parameter_group|✔|
-|aws_db_instance_invalid_type||
+|[aws_db_instance_invalid_type](aws_db_instance_invalid_type.md)||
 |aws_db_instance_invalid_vpc_security_group|✔|
 |aws_elasticache_cluster_invalid_parameter_group|✔|
 |aws_elasticache_cluster_invalid_security_group|✔|

docs/rules/aws_db_instance_invalid_type.md

@@ -0,0 +1,39 @@
+# aws_db_instance_invalid_type
+
+Disallow using invalid type.
+
+## Example
+
+```hcl
+resource "aws_db_instance" "default" {
+  allocated_storage    = 20
+  engine               = "mysql"
+  engine_version       = "5.7"
+  instance_class       = "db.t1.micro" // invalid type!
+  name                 = "mydb"
+  username             = "foo"
+  password             = "bar"
+  db_subnet_group_name = "my_database_subnet_group"
+  parameter_group_name = "default.mysql5.6"
+}
+```
+
+```
+$ tflint
+1 issue(s) found:
+
+Warning: "db.t1.micro" is previous generation instance type. (aws_db_instance_previous_type)
+
+  on template.tf line 5:
+   5:   instance_class       = "db.t1.micro" // invalid type!
+
+```
+
+## Why
+
+Apply will fail. (Plan will succeed with the invalid value though)
+
+## How To Fix
+
+Select valid type according to the [document](https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/Concepts.DBInstanceClass.html#Concepts.DBInstanceClass.Types)
+